1. Field of the Invention
The present invention relates to curable inks, more particularly curable inkjet inks and their use in inkjet printing methods for toys and food packaging applications.
2. Description of the Related Art
In inkjet printing, tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving a print head across the ink-receiver or vice versa or both.
When jetting the inkjet ink onto an ink-receiver, the ink typically includes a liquid vehicle and one or more solids, such as dyes, pigments and polymers. Ink compositions can be roughly divided in:                water-based, the drying mechanism involving absorption, penetration and evaporation;        solvent-based, the drying primarily involving evaporation;        oil-based, the drying involving absorption and penetration;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification; and        UV-curable, in which drying is replaced by polymerization.        
It should be clear that the first three types of ink compositions are more suitable for an absorbing ink-receiver, whereas hot melt inks and UV-curable inks can also be printed on non-absorbing ink-receivers. Due to thermal requirements posed by hot melt inks on the substrates, especially radiation curable inks have gained the interest of the industry in inkjet printing applications.
Migrateable residues in cured layers of inkjet ink on toys or packaging of foodstuffs may present a health risk and consequently they should be kept to an absolute minimum. In general, UV-curable inks contain colorants, monomers, photoinitiators and polymerization synergists. Known measures to reduce extractables of the photoinitiating system from cured ink layers include the use of polymeric or co-polymerizable photoinitiators and synergists instead of the usual low molecular weight compounds.
For example, US 2006014848 (AGFA) discloses radiation curable inkjet inks including a polymeric co-initiator including a dendritic polymer core with at least one co-initiating functional group as an end group. Aliphatic amines and aromatic amines are included as suitable co-initiating functional groups. The dendritic polymeric architecture allows to obtain low extractables and at the same time to minimize the increase in viscosity of the ink.
The colorants used in curable inkjet inks can be dyes, but are generally colour pigments which together with a polymeric dispersant attached to the surface of the pigment are usually very difficult to extract. The remaining problem for extractables includes the monomers. The use of polymerizable oligomers or crosslinkable polymers instead of low molecular weight monomers is only possible up to a certain amount in the ink due to limitations of inkjet printing requiring the inks to possess a low viscosity at the jetting temperature.
In general, the curable inkjet inks are cured by radiation. Thermal curing and electron beam curing of inkjet inks are alternatives for the more preferred radiation curing, more particularly UV-radiation curing. The polymerization mechanism is usually either free radical polymerization or cationic polymerization. There is widespread belief that cationic inkjet inks would be more suitable for food packaging applications. Cationic inkjet inks tend to polymerize slower than free radical polymerizable inkjet inks but to a larger extent. This means that free radical inkjet inks polymerize much faster but the cured image layer contains more extractables, i.e. unreacted monomers.
U.S. Pat. No. 6,803,112 (SUN CHEMICAL) discloses a method for producing a low-extractable film packaging from an actinic radiation curable aqueous composition containing a water soluble compound having at least one α,β-ethylenically unsaturated, radiation polymerizable group and water as essential components carried out by applying the aqueous composition to a surface which is then irradiated in a single step with actinic radiation in the presence of the water thereby forming a cured film wherein less than 50 ppb of the water soluble compound or its residual components are extractable by a food simulant.
The volatility of some of these monomers in curable inkjet inks also contribute to unpleasant odors from printed matter. For non-food printing applications, these unpleasant odors have been camouflaged by addition of deodorizers. For example, US 2005287476 (KONICA MINOLTA) discloses photocurable compositions including a photopolymerizable compound, a photoinitiator and a compound selected from the group consisting of a deodorizer, a perfume and an antioxidant. Also EP 1721943 A (FUJI) discloses the use of fragrances in a curable ink.
US 2003199655 (NIPPON CATALYTIC CHEM) discloses an activated energy ray-curable ink composition for ink-jet printing including a diluent according to Table 1 containing substantially VEEA and monofunctional photoinitiator.
WO 2006/085992 A (HEXION) discloses a radiation curable inkjet ink including a radiation curable composition including about 0.1 to about 15 wt. % of an ethylenically unsaturated mono functional monomer, about 30 to about 80 wt. % of an ethylenically unsaturated difunctional monomer and may further include VEEA and monofunctional photoinitiator.
U.S. Pat. No. 6,310,115 B1 (AGFA) discloses ultraviolet curable ink compositions for ink jet printing including an ultraviolet curable monomer having a vinylether function and a (meth)acrylate function.
Therefore, it would be desirable to have curable inkjet inks that combine the best of both worlds, i.e. the high curing speed of free radical inkjet inks and the complete curing of cationic curable inkjet inks. Furthermore, a need continues to exist for radiation-curable inkjet inks that do not cause bad smell without adding deodorizers or perfumes.